DIII-D Papers

FY18-Q4:

The article “Event hazard function learning and survival analysis for tearing mode onset characterization” by K. E. J. Olofsson et al. (http://iopscience.iop.org/article/10.1088/1361-6587/aac662) was published in Plasma Phys. Control. Fusion. It describes a new flexible approach to event prediction and characterization based on a mixture of machine learning and survival analysis techniques. It is found that a particular tearing mode delta-prime proxy does not contribute to the likelihood of tearing mode onset in a particular class of DIII-D plasmas.

The paper “2D imaging of helium ion velocity in the DIII-D divertor’ by C. Samuell et al. was recently published in Phys of Plasmas (https://doi.org/10.1063/1.5017999). It details a two-dimensional comparison of parallel ion velocities (He+) measured in the divertor using Coherence Imaging Spectroscopy (CIS) and simulated using the UEDGE fluid modeling code. Using pure helium plasmas enabled main-ion measurements close to the divertor target and at low temperatures. These measurements displayed excellent agreement with simulations near the divertor target where He+ is predicted to be the main-ion species and where electron-dominated physics dictates the parallel momentum balance. Upstream near the X-point where He+ is a minority species and ion-dominated physics plays a more important role, there is an underestimation of the flow velocity magnitude by a factor of 2-3 indicating that further investigation the role of ions in determining scrape-off-layer physics is required.

The article “Multi-scale transport in the DIII-D ITER baseline scenario with direct electron heating and projection to ITER” by B. A. Grierson et al was published in Phys. Plasmas (https://doi.org/10.1063/1.5011387) tested the TGLF transport model in DIII-D ITER baseline conditions contrasting electron dominant vs. ion dominant heating. TGLF successfully reproduced the observed modifications in the profile structure and fluctuations associated with changing the heating mix. While the electron temperature profile scale length change was minimized, the effect of increasing Te/Ti to near 1 and reducing the density and collisionality by electron heating produced fluctuations from large to small spatial scales, flattening of the ion temperature profile and an increase of the electron density scale length outside the ECH deposition location. Projecting these results to ITER with TGLF indicates that multi-scale transport processes will also be active, and result in a moderately peaked density profile. At the low gyro-Bohm normalized fluxes in ITER, even moderate rotation shear has a beneficial effect.

The paper “AORSA full wave calculations of helicon waves in DIII-D and ITER” by C. Lau et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aab96d). This paper presents comparison between the AORSA full wave code and GENRAY ray tracing code for helicon current drive applications on example DIII D and ITER scenarios. The paper shows good quantitative agreement for current drive and power absorption for GENRAY and AORSA simulations in the core. The paper also uses AORSA to estimate possible helicon power losses in the SOL at various SOL densities.

The paper “Effects of divertor geometry on H-mode pedestal structure in attached and detached plasmas in the DIII-D tokamak” by H.Q. Wang et al. has been published in Nuclear Fusion DOI: https://doi.org/10.1088/1741-4326/aacbde. Compared to the open divertor, the more closed divertor results in less pedestal fueling, leading to lower pedestal density and higher pedestal temperature in both attachment and detachment. This leads to a wider pedestal than the scaling and theoretical expectation near divertor detachment, and causes significant shift between the density and temperature pedestals. Based on these effects the more closed divertor facilitates the achievement of divertor detachment while retaining high-performance pedestal.

FY18-Q3:

The article “Disruption prediction investigations using Machine Learning tools on DIII-D and Alcator C-Mod” by C. Rea et al was published in Plasma Phys. Control. Fusion (https://doi.org/10.1088/1361-6587/aac7fe). The paper discusses the application of the Machine Learning algorithm called Random Forests for disruption prediction through a comparative study between DIII-D and Alcator C-Mod. The performances of the different Random Forest classifiers are discussed in terms of several metrics; the overall model accuracies are above 97% when identifying a 'far from disruption' and a 'disruptive' phase for disrupted discharges. The Forests are intrinsically different in their capability of predicting disruptive behavior, with C-Mod predictions comparable to random guesses and DIII-D predictions showing a recall index of ~0.72. The portability of the developed algorithm is also tested across the two devices, by using DIII-D data for training the forests and C-Mod for testing and vice versa.

“Dynamic divertor control using resonant mixed toroidal harmonic magnetic fields during ELM suppression in DIII-D” by M. Jia et al was published in Phys. Plasmas (https://doi.org/10.1063/1.5019777). Experiments using Resonant Magnetic Perturbations (RMPs), with a rotating n=2 toroidal harmonic combined with a stationary n=3 toroidal harmonic, have validated predictions that divertor heat and particle flux can be dynamically controlled while maintaining Edge Localized Mode (ELM) suppression in the DIII-D tokamak. Strong changes in the three-dimensional heat and particle flux footprint in the divertor were observed during the application of the mixed toroidal harmonic magnetic perturbations. It agrees well with modeling of the edge magnetic field structure using the TOP2D code, which takes into account the plasma response from the MARS-F code. These results expand the potential effectiveness of the RMP ELM suppression technique for the simultaneous control of divertor heat and particle load required in ITER.

The paper “Physics of increased edge electron temperature and density turbulence during ELM-free QH-mode operation on DIII-D” by C. Sung et al was published in Physics of Plasmas (https://doi.org/10.1063/1.5017964). In this paper, it was found that the increase of turbulence amplitude at near the edge region was correlated with the transition in ELMing to ELM-free phase in the QH-mode plasma. Profile analysis suggests that the decrease in velocity shear, which suppresses turbulence, during ELM-free phase, rather than changes in profile gradients, is responsible for this increased turbulence amplitude. Linear stability analysis also consistently suggests that larger decrease in velocity shear than the changes in linear growth rate can explain this increased turbulence. These results may indicate that the increased turbulence in near the edge of QH-mode can be understood primarily from the changes in turbulence drive (profile gradients) and suppression (velocity shear) terms, which can be a base to develop predictive capability for QH-mode plasmas in the future.

The paper “Implementing a finite-state off-normal and fault response system for disruption avoidance in tokamaks” by N.W. Eidietis, et al., was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aab62c). The paper presents the design of a finite-state off-normal and fault response (ONFR) system implemented in the KSTAR and DIIII-D realtime plasma control systems that provides four critical features of a robust event handling system: sequential responses to cascading events, event recovery, simultaneous handling of multiple events and actuator prioritization. Experimental demonstrations of the ONFR algorithm on the DIII-D and KSTAR tokamaks are presented. In the most complex demonstration, the ONFR algorithm asynchronously applies ‘catch and subdue’ electron cyclotron current drive (ECCD) injection scheme to suppress a virulent 2/1 neoclassical tearing mode, subsequently shuts down ECCD for machine protection when the plasma becomes over-dense, and enables rotating 3D field entrainment of the ensuing locked mode to allow a safe rampdown, all in the same discharge without user intervention.

The paper “Relationship between locked modes and thermal quenches in DIII-D”, by R. Sweeney et al., was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aaaf0a). After a rotating tearing mode locks and the H-mode pedestal is lost, a further collapse of the electron temperature from the edge to near the q=3/2 surface is observed, and attributed to the overlap of edge n=1 locked islands. The poloidal alignment of the O-points at the outboard or inboard mid-plane also appears to play a role in triggering the collapse. Time-evolving a discharge using the NIMROD nonlinear MHD code shows the production of an edge annulus of stochastic fields, and a partial thermal collapse, though the collapse does not reach the q=3/2 surface as observed experimentally. These partial collapses can recover if the island widths decay or their poloidal alignment changes, or they can be followed by a core collapse, resulting in a full thermal quench. This partial collapse is observed to be the first stage in most disruptions following the locking of a tearing mode, and preliminary ideas for avoiding it are proposed.

The paper “Resolving runaway electron distributions in space, time, and energy” by C. Paz-Soldan et al was published in Phys. Plasmas (https://doi.org/10.1063/1.5024223). Areas of agreement and disagreement with present-day models of runaway electron (RE) evolution are revealed by measuring MeV-level bremsstrahlung radiation from runaway electrons (REs) with a pinhole camera. Spatially resolved measurements localize the RE beam, reveal energy-dependent RE transport, and can be used to perform full two-dimensional (energy and pitch-angle) inversions of the RE phase-space distribution. Energy-resolved measurements find qualitative agreement with modeling on the role of collisional and synchrotron damping in modifying the RE distribution shape. Temporally resolved measurements find anomalous RE loss, which is found to be largest at low energy. Possible roles for kinetic instability or spatial transport to resolve these anomalies are discussed.

The paper “First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks,” by D. A. Spong et al. was recently published in Phys. Rev. Lett. (https://doi.org/10.1103/PhysRevLett.120.155002). This paper presents results and interpretation of experiments carried out on DIII-D under the Frontier Science program which made the first detailed observations of runaway electron driven whistler instabilities. The paper includes comparison of the frequency scaling of the waves against the whistler dispersion relation, presents stability threshold measurements, discusses the destabilizing wave-particle resonances, gives simulations of the mode structure and explanations for the observed discrete frequency bands, and presents results for the nonlinear limit cycles and scattering of runaways (diagnosed through electron cyclotron emission) by the whistlers.

The review paper “A review of direct experimental measurements of detachment” by J Boedo et al. was published in Plasma Phys and Control. Fusion (https://doi.org/10.1088/1361-6587/aaa2ec). Detached divertor plasmas feature strong radial and parallel gradients of density, temperature, electric fields and flow over the divertor volume and therefore, sampling the divertor plasma directly provides crucial knowledge to the interpretation and modeling efforts. Contribution of diagnostics that directly sample the plasma to the advancement of knowledge of the physics of detachment and detached divertors are reviewed, with a focus on wall probes, scanning probes, retarding field analyzers and Thomson scattering in the divertor region and also include the contribution of measurements away from the divertor that provide insight on how divertor detachment affects core, edge or pedestal conditions.

The paper “H2 optimal control techniques for resistive wall mode feedback in tokamaks” by M. Clement et al was published in Nucl. Fusion 58 (https://doi.org/10.1088/1741-4326/aaaecd). This paper describes how an advanced RWM feedback algorithm using DIII-D's external 3D coils was developed and tested using MHD spectroscopy and simulations with the VALEN RWM model. This algorithm proved more effective than the conventional proportional control algorithm normally in use when used with external coils. Furthermore, AT scenario discharges which had only previously been accessible with internal 3D coil feedback were replicated using external coils and this new algorithm.

The article “Advances in Low-Temperature Tungsten Spectroscopy Capability to Quantify DIII-D Divertor Erosion” by T. Abrams et al. was published in IEEE Transactions on Plasma Science (https://doi.org/10.1109/TPS.2018.2797691). This paper describes recent enhancements to the DIII-D WI and WII diagnostic capabilities to further validate the PMI physics of high-Z erosion and re-deposition. Notably, a novel method was developed for the DIII-D high temporal resolution filterscopes to distinguish between W I light and background contamination using two different bandpass filters with different widths but the same center wavelength. This is important due to the relatively weak intensity of W lines in the visible spectrum.

The paper “Quasistationary Plasma Predator-Prey System of Coupled Turbulence, Drive, and Sheared E×B Flow During High Performance DIII-D Tokamak Discharges ”, by K. Barada et al was recently published in Phys. Rev. Lett. (https://doi.org/10.1103/PhysRevLett.120.135002), which describes a long-lived predator-prey type limit cycle oscillation (LCO) regime which follows immediately after EHOs disappear in wide-pedestal QH-mode. A radially inward propagation of E×B velocity perturbation was found to be the key to generation of E×B shear variations necessary to regulate the edge turbulence. Transport relevant edge parameters are found to oscillate at the LCO frequency indicating a significant role of the observed LCOs in modulated edge transport in these near-zero torque ELM-free plasmas.

The review article “Plasma Detachment in Divertor Tokamaks” by A.W. Leonard was published in Plasma Phys. Control. Fusion (https://doi.org/10.1088/1361-6587/aaa7a9). This paper summarizes experimental observations of divertor detachment in terms of transport and dissipation of power, momentum and particle flux along the open field lines from the midplane to the divertor. Asymmetries in detachment between the inboard and outboard divertor are characterized and found to be primarily driven by plasma electric field drifts. The effect of divertor geometry on detachment and active control of detachment is also summarized. Finally, the compatibility of detached divertor operation with high performance core plasmas is examined.

The paper “Exploratory Machine Learning Studies for Disruption Prediction Using Large Databases on DIII-D”, by C. Rea and R.S. Granetz was published in Fusion Sci. Technol. (https://doi.org/10.1080/15361055.2017.1407206). A large set of disruptive and non-disruptive discharges is used to develop a disruption classification algorithm using a Machine Learning technique called Random Forests. Binary and multi-class classification schemes are tested and, depending on the formulation of the problem, an overall accuracy up to 90% is demonstrated, approaching 97% when identifying a disruptive phase (tdisrupt - t < 0.35 s) in discharges that eventually disrupt.

The paper “Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D” by D. Shiraki et al, was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aab0d6). This paper describes the dissipation of fully-avalanched post-disruption runaway electron (RE) beams by shattered pellet injection and by equivalent massive gas injection, with the two techniques leading to comparable dissipation suggesting that pellet fragments are rapidly ablated by the relativistic electrons before significant radial penetration into the runaway beam can occur. High-Z neon and argon injection are shown to enhance current dissipation by pitch-angle scattering, while injection of deuterium is shown to purge high-Z impurities and reduce dissipation, as has been used in several subsequent DIII-D runaway electron experiments to control RE plateau plasma conditions. Injecting high-Z/deuterium mixtures shows that deuterium levels as low as 10% may reduce the resulting dissipation, suggesting that future mitigation schemes based on secondary injections may need to completely eliminate deuterium for optimal mitigation.

FY18-Q2:

The paper “Developing Physics Basis for the Snowflake Divertor in the DIII-D tokamak” by V. A. Soukhanovskii et al. was published in Nucl Fusion (https://doi.org/10.1088/1741-4326/aaa6de). The paper summarized results from snowflake divertor experiments in DIII-D. The snowflake divertor enabled significant manipulation of divertor heat transport for heat spreading and reduction in attached and radiative divertor regimes, between and during edge localized modes, while maintaining good H-mode confinement.

The paper “Modelling of N seeding experiments in the ASDEX Upgrade tokamak,” by L. Casali, et al., was published in Physics of Plasmas (https://aip.scitation.org/doi/full/10.1063/1.5019913). This paper investigates the complex mechanisms which govern the behavior of radiation and impurities in presence of ELMs. Time dependent simulations have been carried out with the ASTRA transport code coupled to the impurity transport code STRAHL featuring the self-consistent interplay between background plasma, impurity transport, radiation and ELM-induced transport. ELMs are modeled based on the two different assumptions of a diffusive and a convective transport respectively. The work highlights the importance of non-coronal effects through the ELM-induced transport for low-Z impurities. High ELM frequency is required to avoid radiation collapse by tungsten accumulation in the simulations which is consistent with the experiments.

The letter “Numerical exploration of non-axisymmetric divertor closure in the small angle slot (SAS) divertor at DIII-D” was published in Nuclear Fusion (https://doi.org/10.1088/1741-4326/aaaf99). This paper examines the effects of toroidally localized changes in divertor closure due to misalignment, using the 3-D code EMC2-EIRENE and a simple model where toroidal gaps are placed in the SAS slot geometry. The main effect observed is to raise the required separatrix density for toroidally symmetric detachment by 10-15%. These simulations showed that, even in the lower density cases while the plasma may be both attached and detached at different toroidal locations, the non-axisymmetric divertor conditions have little to no impact on upstream conditions.

The paper “Feedforward and feedback control of locked mode phase and rotation in DIII-D with application to modulated ECCD experiments”, by W. Choi et al, has been published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aaa6e3). It is known that the toroidal phase of a locked or near-locked m/n=2/1 mode can be controlled by applied resonant magnetic perturbations. The article demonstrates locking avoidance and rapid entrainment of the mode when the rotating fields are applied in feedforward. It also demonstrates precise control of the mode phase as a function of time by using a novel feedback algorithm. This allowed for simple and reproducible synchronization with modulated electron cyclotron current drive, for basic studies of island stability and future applications to efficient mode stabilization.

The paper “Simulation of density fluctuations before the L-H transition for Hydrogen and Deuterium plasmas in the DIII-D tokamak using the BOUT++ code”, by Y.M. Wang et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa9f7d). The six-field two-ﬂuid BOUT++ code has been used to simulate density ﬂuctuations. The equilibriums are generated by experimental measurements for both Deuterium (D) and Hydrogen (H) plasmas at the lowest densities of DIII-D L-H transition experiments. In linear simulations, the unstable modes are found to be resistive ballooning modes with the most unstable mode number n = 30 or kθρi ∼ 0.12. The spectrum of the density fluctuation in non-linear simulations can be well compared with the beam emission spectroscopy (BES) measurements on the DIII-D tokamak. The electric field scan and ion mass scan results show that the dual-mode results primarily from differences in the profiles rather than the ion mass.

“Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on DIII-D”, by Z. Yan et al, was published in Nucl. Fusion 57 (https://doi.org/10.1088/1741-4326/aa82c9). This paper describes the experimental observations of single and double bands of low-wavenumber turbulence near the edge of H and D plasmas across L-H transition. The double bands are only observed when the power threshold is lower and the turbulence Reynolds and ExB shear is larger. The increased edge fluctuations, increased flow shear, and the double- band nature of edge turbulence correlating with lower PLH may account for the strong isotope and density dependencies and support current L-H transition theories, but suggest a complex behavior that can inform a more complete model of the L-H transition threshold.

The paper “Nonlinear MHD simulations of QH-mode DIII-D plasmas and implications for ITER high Q scenarios” by F Liu et al was published in Plasma Phys. Control. Fusion (https://doi.org/10.1088/1361-6587/aa934f). This paper shows that a sufficient edge plasma current is required for driving unstable/saturate an EHO and to obtain a stationary QH-mode, and that at lower plasma edge current levels the higher pedestal pressures drive the plasma into ballooning instability, i.e. into the ELMy H-mode domain. In QH-mode E×B rotation plays an important role by destabilizing the toroidal mode n=2 but stabilizing high-n modes (n>5). Simulations of ITER Q=10 plasmas show that the pedestal currents are large enough to destabilize low-n kink-peeling modes but need further study to improve the predictive capability towards ITER.

“Turbulence evolution and transport behavior during current ramp-up in ITER-like plasmas on DIII-D”, by G.R. McKee et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa78b8). Turbulence characteristics and behavior during the current ramp-up phase of scaled ITER-like discharges were measured on DIII-D with BES and other fluctuation diagnostics in an effort to identify why transport models exhibit notable inaccuracies in predicting the electron temperature profile during this phase. Measurements demonstrated unique turbulence and transport behavior during the dynamic ramp-up phase that may partially explain these modeling discrepancies: co-existing and counter-propagating low-wavenumber modes are observed in the outer radial regions of these plasmas (rho>0.9), suggesting a complex multi-modal instability structure; transient time windows exhibit extremely low localized fluctuations; and numerous low-order rational q-surfaces reduce local turbulence and increase electron temperature as they enter the plasma and propagate radially outward. Models don’t exhibit changes to linear growth rates or frequencies in response to proximity to low-order surfaces, suggesting a mechanism for improving model comparisons and predictions. Accurately predicting the evolution of the electron temperature profile is crucial as it impacts current diffusion rates, q-profile evolution, inductance, and, ultimately, poloidal power supply requirements, since much of the solenoid ohmic flux will be consumed during the initial ~100 second current ramp.

“Transport modeling of the DIII-D high βp scenario and extrapolations to ITER steady-state operation”, by J. McClenaghan et al, was published in Nucl. Fusion (https://dx.doi.org/10.1088/1741-4326/aa79ca). A high bootstrap fraction, high βp scenario is found to be near an ITB formation threshold, and either strong negative central magnetic shear or rotation in a high bootstrap fraction are found to successfully provide the turbulence suppression required to achieve Q=5. TGYRO transport modeling is carried out under systematic variations of the toroidal rotation and the core q-profile to better understand requirements on ITB formation. DIII-D modeling predictions of the electron transport in the high βp scenario improve as q95 approaches levels similar to typical existing models of ITER steady-state and the ion transport is turbulence dominated.

“Increased electron temperature turbulence during suppression of edge localized mode by resonant magnetic perturbations in the DIII-D tokamak”, by C. Sung et al, has been published in Phys. Plasmas (https://doi.org/10.1063/1.4999785). This article investigates the dynamic changes in turbulence, profiles, and heat transport as resonant magnetic perturbations (RMP) are applied in the H-mode discharge. This paper reports the first observation of increased long wavelength electron temperature as well as density turbulence at top of the pedestal during RMP edge localized mode (ELM) suppression. This occurs after the ELMs are suppressed and in conjunction with gradient drive changes rather than with the initial RMP application. This also indicates that the changes in turbulence amplitude at top of the pedestal do not result in the RMP density pump-out, which occurs right after the RMP application.

“A model of the saturation of coupled electron and ion scale gyrokinetic turbulence” by G.M. Staebler et al was published in Nucl. Fusion (https://dx.doi.org/10.1088/1741-4326/aa6bee). The model is based on the saturated intensity spectrum of electric potential fluctuations from a set of multi-scale (electron plus ion scales) GYRO simulations of C-MOD L-mode discharges by N. Howard. A new paradigm of zonal flow mixing (not shearing) is deduced from the properties of the potential spectrum. The zonal flows driven by the ion scale turbulence impacts the saturation of electron scale turbulence. A formula for the threshold for enhanced electron scale “streamer” transport is determined from the simulations and built into the new saturation model.

The paper “Impact of neoclassical tearing mode-turbulence multi-scale interaction in global confinement degradation and magnetic island stability,” by L. Bardoczi et al. was published in Phys. Plasmas 24, 122503 (2017) as a Featured Article (https://doi.org/10.1063/1.5004987). The paper reports measurements of temporarily increased turbulence outside the region of growing NTM magnetic islands when the plasma stored energy is decreasing, which indicates that the increased fluxes leading to the soft-beta limit result from magnetic island - turbulence multi-scale interaction. Simultaneous reduction of turbulence inside the island region makes the islands more unstable. This suggests that the growth of harmful large islands can be avoided by driving turbulence at the mode rational surface (e.g. by ECH) while the islands are still small.

The paper “Joint DIII-D/EAST research on the development of a high poloidal beta scenario for the steady state missions of ITER and CFETR” by Garofalo et al, (https://doi.org/10.1088/1361-6587/aa8c9d) was published in Plasma Phys. Control. Fusion. This paper discusses the progress achieved in research activities (experiments and simulations) performed in the last few years through an international collaboration between DIII-D and EAST scientists, aiming to develop the high poloidal beta regime as a basis for the steady state operation of a tokamak fusion reactor.

A paper on “Tungsten erosion by unipolar arcing in DIII-D” during the Metal Rings Campaign by I.Bykov et al. was published in Physica Scripta (https://doi.org/10.1088/1402-4896/aa8e34). It presents the results of in-situ and post mortem studies of the W inserts used in DIII-D and investigates the significance of arcing for the net campaign-averaged W erosion. The work shows that arcing only occurred during ELMs and disruptions and reveals a threshold character of arc ignition with almost no arcing at deposited heat flux < 2MW/m2.

The paper “Tests of a two-color interferometer and polarimeter for ITER density measurements” by M.A. Van Zeeland et al (http://iopscience.iop.org/article/10.1088/1361-6587/aa8c49) was published in Plasma Phys. and Control. Fusion. This paper discusses the design, construction and testing of a full-scale ITER toroidal interferometer and polarimeter (TIP) prototype including an active feedback alignment system. The TIP prototype is constructed around a 10.59 micron CO2 laser and a 5.22 micron Quantum Cascade Laser and the prototype beam path incorporates translation stages to simulate ITER motion through a bake cycle as well as other sources of motion or misalignment. It was found that even in the presence of significant motion, the TIP prototype can meet ITER’s density measurement requirements over 1000s shot durations with demonstrated phase resolution of 0.06 deg. and 1.5 deg. for the polarimeter and vibration compensated interferometer respectively. Measurements of a pulsed radio frequency device show a line-integrated density resolution of ~3.5×10^17 m^-2. These tests were carried out in preparation for installation of the system on DIII-D.

FY18-Q1:

“Action-angle formulation of generalized, orbit-based, fast-ion diagnostic weight functions”, by L. Stagner and W. W. Heidbrink, was recently published in Phys. Plasmas (https://doi.org/10.1063/1.4990391). This paper provides the theoretical framework needed for calculating orbit weight functions. Using orbit weight functions the forward model of the diagnostic can be linearized without sacrificing accuracy. The linearized form of the forward model will allow us to do Orbit Tomography which is a method of inferring the full fast-ion distribution function from experimental measurements. Orbit weight functions for the Fast-ion D-alpha (FIDA), Neutral Particle Analyzer (NPA), and Neutron diagnostics are also derived.

“Modeling of 3D magnetic equilibrium effects on edge turbulence stability during RMP ELM suppression in tokamaks” by R.S. Wilcox et al was published in Nuclear Fusion (https://doi.org/10.1088/1741-4326/aa7bad). Small 3D perturbations to the magnetic field in DIII-D result in large modulations to density fluctuation amplitudes in the pedestal, which are shown using Doppler backscattering measurements to vary spatially by a factor of 2. Helical perturbations of equilibrium density within flux surfaces have previously been observed in the pedestal of DIII- D plasmas when 3D fields are applied and were correlated with density fluctuation asymmetries in the pedestal. These intra-surface density and pressure variations are shown through two fluid MHD modeling using the M3D-C1 code to result from the misalignment of the density and temperature equilibrium iso-surfaces in the pedestal region. The resulting pedestal density, potential, and turbulence asymmetries within flux surfaces near the separatrix may be at least partially responsible for several poorly understood phenomena that occur with the application of 3D fields in tokamaks, including density pumpout, the modification of 3D heat flux structures in the scrape-off layer, and an increase in heating power required to enter H-mode.

“Understanding and predicting profile structure and parametric scaling of intrinsic rotation”, by W. X. Wang et al was published in Phys. Plasmas (https://doi.org/10.1063/1.4997789). This paper shows that turbulent fluctuation-driven residual stress (a non-diffusive component of momentum flux) along with diffusive momentum flux can account for both the shape and magnitude of the observed intrinsic toroidal rotation profile. The model predictions of core rotation based on global gyrokinetic simulations agree well with the experimental measurements of main ion toroidal rotation for a set of DIII-D ECH discharges. The validated model is further used to investigate the characteristic dependence of residual stress and intrinsic rotation profile structure on the multi-dimensional parametric space covering the turbulence type, q-profile structure, and up-down asymmetry in magnetic geometry, with the goal of developing the understanding needed for rotation profile control.

The paper “Core tungsten radiation calibration by small shell pellet injection in the DIII-D tokamak,” by E.M. Hollmann et al was published in Rev. Sci. Instrum. (https://dx.doi.org/10.1063/1.5005170). The paper describes the use of small (OD = 0.8 mm) shell pellets to deliver known quantities (10 ug) of W to the plasma core for checking calibration of spectroscopic estimates of core W concentration. It is found that predicted individual UV line brightnesses are often less accurate than a factor of 2 while total brightness or total soft x-ray brightness (integrating over many lines) are usually better than a factor of 2 for estimating core W concentration.

The paper “Comparison of heat flux measurement techniques during the DIII-D metal ring campaign” by Barton et al was published in Physics Scripta (https://doi.org/10.1088/1402-4896/aa878a). This paper examines the heat fluxes deposited on the metal rings measured by IRTV, Langmuir probes, and thermocouples in order to better characterize the initial conditions of tungsten impurity sourcing in the divertor. We observe a linear relationship between input heating power and deposited heat flux on the rings. A simple heat flux model is developed to get ELM and inter-ELM heat fluxes from the embedded thermocouples. This technique is within a factor of 2 of the IRTV measurements and may be a useful technique when the IRTV view is obstructed (e.g. in closed/SAS divertor geometries).

“Understanding ECH density pump-out in DIII-D H-mode plasmas” by X. Wang et al. Nucl. Fusion 57 (2017) 116046; https://doi.org/10.1088/1741-4326/aa7f99. This paper shows, through time-dependent analysis, that the density pump-out observed in ECH heated plasmas is not related to a change in turbulence regime in the plasma core. We observe that the density pump-out originates around rho~0.8 and that this decrease in the density is linked to an increase in density fluctuations, not a change in turbulence regime. Linear gyrokinetic simulations confirm the increase in turbulence drive. Only after 100ms do the simulations suggest a change in turbulence regime in the plasma core, long after the initial pump-out. As such we can conclude that the ECH driven density pump-out in low collisionality plasmas is not the result of a change in turbulence regime from ITG to TEM.

The paper “The effect of electron cyclotron heating on density fluctuations at ion and electron scales in ITER baseline scenario discharges on the DIII-D tokamak” by Marinoni et al., was published in Nuclear Fusion (http://iopscience.iop.org/article/10.1088/1741-4326/aa8333). This paper examines the effect of ECH on confinement in IBS plasmas by measuring the time evolution of density fluctuations after ECH is turned off at fixed bN. While the modification to the ExB flow shear caused by the removal of ECH is shown to dominate the evolution of ion scale fluctuations and confinement on time scales comparable to the energy confinement time, electron scale modes are seen to respond to the heat flux variation within 20 ms. Linear and non-linear gyro-kinetic simulations predict such modes to be mostly unstable in the outer third of the minor radius, and to produce a prompt particle pinch that could be related to the ECH density pump-out.

The paper “Study of the impact of resonant magnetic perturbation fields on gross tungsten erosion using DiMES samples in DIII-D” by E. Hinson was published in Physica Scripta (https://doi.org/10.1088/1402-4896/aa9002). The paper describes the results of an experiment comparing gross tungsten erosion on samples exposed to OSP sweeps in L-mode with and without RMP. It was found that upon RMP application, lobes evident in divertor camera data and on Langmuir probes increased gross erosion in the RMP cases by no more than 30% above the level observed in unperturbed discharges. A large reduction in gross erosion (50%) was observed in the private flux region at the W sample for one specific toroidal phase of the RMP field.

The paper “Bifurcation of quiescent H-mode to a wide pedestal regime in DIII-D and advances in the understanding of edge harmonic oscillations”, by X. Chen et al was published in Nucl. (https://doi.org/10.1088/1741-4326/aa7531). It reports recent torque ramp experimental investigation of the critical edge ExB shear for exciting and sustaining EHO which regulates the conventional QH-mode. A favorable linear correlation between the critical ExB shear and the pedestal electron collisionality is observed in the initial analysis. The transition from conventional QH-mode to wide-pedestal QH-mode occurs only if the torque is reduced sufficiently, which supports the hypothesis that the decreased edge ExB shear enables destabilization of broadband turbulence which governs the wide-pedestal QH-mode edge. In addition, counter-propagating multi-branches of the low-k components of the broadband turbulence are observed along with more localized intermediate-k component. These observations and understandings advance the physics basis and confidence in QH-mode as a high confinement, ELM-stable operating mode for future burning plasmas where similar collisionality and rotation levels are expected.

A paper on “Modeling non-stationary, non-axisymmetric heat patterns in DIII-D tokamak” by D. Ciro et al was published in Nucl. Fusion (https://dx.doi.org/10.1088/0029-5515/57/1/016017), and describes results on the interaction of a slowly rotating quasi-stationary mode (QSM) with the divertor heat flux just before the onset of locking that results in a disruption. Here, it is shown that a multi-filamentary model of the magnetic field produced by the QSM interacts with the separatrix to generate time-dependent divertor footprint modulations that are in good agreement with the measured divertor heat flux.

The paper “Poloidal radiation asymmetries during disruption mitigation by massive gas injection on the DIII-D tokamak”, by N.W. Eidietis et al was published in Phys Plasmas (http://dx.doi.org/10.1063/1.5002701). This paper describes measurements of poloidal variations in the radiated power emissivity resulting from neon massive gas injection, as a function of the poloidal position of the injectors. The poloidal evolution of the emissivity, including the direction and magnitude of flow, is found to exhibit a strong dependence upon injector poloidal location, although the physical mechanism for the difference is not clear. Thermal quench MHD is found to have little effect upon the poloidal phase of maximum emissivity in experiment or modeling, which is attributed to the slower parallel transport of impurities along field lines in the poloidal versus toroidal direction. Poloidal peaking factors of 1.6 and 2.2 were observed for upper and lower injection, respectively.

The paper “Validation of the model for ELM suppression with 3D magnetic fields using low torque ITER baseline scenario discahrges in DIII-D,” by R.A. Moyer et al., has been published in Phys. Plasmas (http://dx.doi.org/10.1063/1.5000276). This paper reports results of attempts to extend RMP ELM control to more ITER-relevant levels of input torque ~ 1 Nm in ITER Baseline Scenario discharges in DIII-D. At both high (>1) and low (~ 0.15) pedestal collisionality, reducing the input torque below ~ 4 Nm led to a loss of ELM suppression. These results are used to validate the existing model for RMP ELM suppression in which a tearing response at the zero-crossing in the electron perpendicular rotation leads to ELM suppression when this tearing response is close enough to the top of the pedestal to limit the expansion of the pedestal to stable widths for peeling-ballooning modes.

FY17-Q4:

The paper “Increased heat dissipation with the X-divertor geometry facilitating detachment onset at lower density in DIII-D,” by B. Covele et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa7644). X-Divertors have been created and tested for their ability to facilitate detachment at lower densities on DIII-D. Tangential camera and probe data from the lower floor suggest that the X-Divertor detaches more readily than a standard divertor, likely due to an enhanced radiating volume from magnetic flaring near the target. SOLPS confirms this observation qualitatively, showing strong poloidal gradients in electron temperature in the X-Divertor leg, spatially correlated with the flaring effect. X-Divertor detachment at lower densities and higher powers may widen the window for high-performance core operation with acceptable exhaust heat mitigation.

The paper “Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes”, by C.S. Collins at al, was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa720c). Measurements show that some Alfvén eigenmodes (AEs) can be tolerated before significant fast-ion transport occurs, and a sudden rise in transport occurs when multiple, overlapping AEs cause fast-ion orbits to become stochastic. Simulations using the 'kick model' accurately reproduce the measured signals and radial critical beam ion density gradient, highlighting the importance of including the energy and pitch dependence of the fast-ion distribution function in critical gradient models. These studies are being used to validate reduced models and provide the basis for understanding how to avoid AE transport that can undesirably redistribute current, reduce performance, and cause fast-ion losses in tokamaks.

The paper “Self-consistent core-pedestal transport simulations with neural network accelerated models”, by O. Meneghini et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa7776). This paper describes the development of two neural-network (NN) based models as a means to perform a non-linear multivariate regression of theory-based models for the core turbulent transport fluxes, and the pedestal structure. These models are integrated into a predictive workflow that allows prediction with self-consistent core-pedestal coupling of the kinetic profiles within the last closed flux surface of the plasma. The NN paradigm is capable of breaking the speed-accuracy trade-off that is expected of traditional numerical physics models, and provides the missing link towards self-consistent coupled core-pedestal whole device modeling simulations that are physically accurate and yet take only seconds to run.

“An efficient technique for magnetic equilibrium reconstruction with q profile constraints and its application on the EAST tokamak”, by J.P. Qian et al, was recently published Nucl. Fusion 57 (2017) 084001; (https://doi.org/10.1088/1741-4326/aa74eb). This paper describes a technique to efficiently construct plasma equilibria with preset or constrained q profiles by iteratively solving the nonlinear GS equation using a set of dynamic virtual internal magnetic probes. With this technique, model or experimental equilibrium with a desirable q profile can be conveniently constructed by directly constraining the target q profile. Plasma equilibrium reconstruction using experimental ECE q=1 surface information is demonstrated.

The paper “Study of Z scaling of runaway electron plateau final loss energy deposition into wall of DIII-D”, by E. M. Hollmann et al was published in Phys. Plasmas (http://doi.org/10.1063/1.4985086). This paper deals with the total energy deposited into the wall during a runaway electron beam strike into the wall. It is shown that the amount of magnetic energy converted into kinetic energy can be fairly well described by a simple coupled current model. However, for this to work, the loss time into the wall must be known; this is found experimentally to not be constant but vary roughly proportional to the avalanche time scale, resulting in a magnetic - kinetic energy conversion factor which is non-monotonic with increasing impurity content Z

“ELM suppression in helium plasmas with 3D magnetic fields” by T.E. Evans et al was published in Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa7530). This paper discusses results from a set of DIII-D experiments that were the first to demonstrate access to RMP ELM suppression in high purity (>95%) helium plasmas at ITER relevant conditions with edge toroidal rotation frequencies of <10 krad/s and input power levels slightly above those required for an L-H transition. These results are particularly important for providing guidance on obtaining ELM suppression during the early non-active helium plasma operational phase of ITER.

The paper “Compatibility of separatrix density scaling for divertor detachment with H-mode pedestal operation in DIII-D” by Leonard et al was published in Nuclear Fusion (https://doi.org/10.1088/1741-4326/aa778c). This study characterized the midplane separatrix density in response to variations in the core plasma density, particularly the H-mode pedestal density, and the upstream Scrape-Off-Layer heat flux density. The separatrix density just before divertor detachment was found to increase with higher heat flux consistent with a simple scaling model. Ratios of separatrix density to pedestal density of greater than 60% were achieved. The scaling of separatrix density and H-mode power threshold were combined to obtain a minimum ratio of separatrix to pedestal density that will be required for heat flux control in future devices. The scaling highlights the boundary plasma challenge for tokamaks of larger size and higher field. Jul

The paper “Modeling of rapid shutdown in the DIII-D tokamak by core deposition of high-Z material”, by V. A. Izzo and P. B. Parks was published in Phys. Plasmas (http://dx.doi.org/10.1063/1.4990447). Shell-pellet injection, an alternate disruption mitigation concept, is modeled in 3D MHD simulations. The shell-pellet concept consists of a dispersive payload encased in a low-Z shell, and aims to deliver to payload directly to the center of the plasma while minimally perturbing the edge plasma. The simulations support the primary proposed benefits of the concept, which includes the ability to radiate a very large fraction of the core thermal energy without break-up of the outer flux surfaces (leading to heat loss to the divertor) until the thermal quench is nearly complete. The simulations also show very rapid loss of runaway electrons as soon as the outermost flux surfaces are destroyed. The simulations motivate continued experimental efforts to advance this concept.

The paper “Modification of adhered dust on plasma-facing surfaces due to exposure to ELMy H-mode plasma in DIII-D” by I. Bykov et al. was published in Nucl. Mater. and Energy. The paper shows that heating of C and metal dust on W surfaces in ELMy H-mode plasmas in DIII-D leads to reduced remobilization and improved adhesion to the surface. Melting of metal dust leads to coalescence and formation of larger grains. This may complicate removal of dust from plasma facing surfaces in ITER. (https://doi.org/10.1016/j.nme.2017.05.006)

The paper “Investigation of the plasma shaping effects on the H-mode pedestal structure using coupled kinetic neoclassical/MHD stability simulations” by A. Y. Pankin et al was published in Phys. Plasmas. The pedestal widths and heights for DIII-D discharges with different elongations and triangularities are studied in the coupled neoclassical/ideal MHD stability simulations. The pedestal width for the plasma density is found almost independent of triangularity and the pedestal width for the electron temperature is found almost independent of elongation. Different dependences of the pedestal widths and heights on elongation and triangularity for different plasma profiles can provide a mechanism for controlling the bootstrap current in the pedestal and the pedestal stability. (http://dx.doi.org/10.1063/1.4984256)

The article “Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks ” by C. Paz-Soldan et al. has been published in Phys. Rev. Lett. (https://doi.org/10.1103/PhysRevLett.118.255002). This work describes the first measurements of runaway electron distribution functions and dissipation rates with the DIII-D Gamma Ray Imager (GRI) system as plasma parameters are varied. This enabled validation of several key theoretical predictions, such as the appearance of non-monotonic features at energies consistent with theory, as well as the shift of the distribution function to lower energy with increasing collisional and synchrotron damping. Observed discrepancies will also guide improvements to runaway electron dissipation models.

The joint DIII-D/EAST paper “Development of high poloidal beta, steady-state scenario with ITER-like tungsten divertor on EAST”, by A.M. Garofalo et al. has been published in the journal Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa7186). This paper discusses recent experiments on EAST that have achieved the first long pulse H-mode (61 s) with zero loop voltage and an ITER-like tungsten divertor, and have demonstrated access to broad plasma current profiles by increasing the density in fully-noninductive lower hybrid current-driven discharges. Using the newly commissioned polarimeter-interferometer diagnostic for q-profile measurements, these experiments also provide a strict test of LHCD deposition models. Modeling with GENRAY-CQL3D reproduces the trend of current profile broadening with higher density.

“Total fluid pressure imbalance in the scrape-off layer of tokamak plasmas”, by R.M. Churchill et al has been published in Nucl. Fusion (http://doi.org/10.1088/1741-4326/aa5fb1). Understanding variation of plasma quantities from upstream to the divertor is critical for planning future fusion machines. Kinetic simulations of a DIII-D H-mode plasma using the XGCa code showed that the total pressure in the near-SOL does not vary as would be expected based on simplified, fluid-based expressions. Direct inclusion of terms for neutral drag and parallel viscosity utilizing a CGL assumption do not resolve the pressure discrepancy, and suggest further investigations to kinetic effects which may explain the simulation observation.

The paper “Experimentally-based ExB drifts in the DIII-D divertor and SOL calculated from integration of Ohm’s Law using Thomson Scattering measurements of Te and ne” by P. Stangeby et al was published in Nuclear Materials and Energy. Experimentally-derived 2D spatial distributions of cross field drift velocities are obtained from 2D Thomson scattering measurements of T_e and n_e in the divertor and SOL of DIII-D. In contrast with the method that has been used on DIII-D where the 2D distribution of plasma potential V_plasma is obtained from measurements of the probe floating potential of reciprocating probes, the present method does not require insertion of a probe into the plasma and can therefore be used in high power discharges. The 2D spatial distribution of V_plasma is calculated from Ohm’s Law for the parallel electric field E_par along each flux tube. E_radial and E_poloidal are calculated from the spatial derivatives of V_plasma, providing the 2D spatial distributions of cross field drift velocities. (http://dx.doi.org/10.1016/j.nme.2017.03.021)

The paper “Changes in divertor conditions in response to changing core density with RMPs” by Briesemeister et al. has been published in Nuclear Fusion. In this paper, the effects of resonant magnetic perturbations (RMPs) on the divertor plasma temperature, density, heat and particle flux are explored for different core densities. Increases in the heat flux to the divertor observed when the RMPs are applied are shown to be eliminated when gas puffing is used to compensate for the core density pump-out. The electron temperature at the target plate, including that in the RMP induced lobe structures is shown to decrease with increasing density in both measurements and EMC3-Eirene simulations. These results are favorable for the compatibility of RMPs with the need for low electron temperatures to protect divertor surfaces in future devices. (https://doi.org/10.1088/1741-4326/aa70bd).

The paper “SOL effects on the pedestal structure in DIII-D discharges” by A. Sontag et al. was published in Nuclear Fusion. This paper examines how the change in grad-B drift direction changes the flows in the pedestal and can affect the pedestal height. (https://dx.doi.org/10.1088/1741-4326/aa6cb6).

“Impact of toroidal and poloidal mode spectra on the control of non-axisymmetric fields in tokamaks” by M. J. Lanctot et al. was published in Physics of Plasmas (http://dx.doi.org/10.1063/1.4982688). Experimental determination of the threshold for driven magnetic reconnection using externally applied n=2 fields measured in several tokamaks shows that plasmas are not necessarily more sensitive to applied n=1 magnetic fields. Instead, the primary need for n=1 error field correction in present experiments can be attributed to the preferential coupling between certain poloidal harmonics of an error field and the range of safety factor profiles typically generated in tokamak plasmas. The dependence of the threshold on relevant plasma parameters is reported and implications for ITER discussed.

FY17-Q3: 27 papers described here

Several members of the DIII-D team participated in the 44th European Physical Society Conference on Plasma Physics which was held in Belfast, Northern Ireland from June 26-30, 2017. Fifteen presentations were made including two invited talks (by Andrea Garofalo and Feng Liu).

The paper “Controlling marginally detached divertor plasmas” by D. Eldon, et al. was published in Nuclear Fusion. The paper describes the development of a control system used for finding and supplying the necessary amount of gas puffing to produce divertor detachment that has been demonstrated at DIII-D. This control system has also been used to investigate the properties of the Te cliff observed at detachment onset (http://dx.doi.org/10.1088/1741-4326/aa6b16).

The paper “Shrinking of core neoclassical tearing mode magnetic islands due to edge localized modes and the role of ion-scale turbulence in island recovery in DIII-D” by L. Bardóczi, T. L. Rhodes, T. A. Carter, R. J. La Haye, A. Bañón Navarro, and G. R. McKee has been published in Physics of Plasmas (http://dx.doi.org/10.1063/1.4985078). Experimental signature of ion-scale turbulence accelerating the recovery of NTMs after they have been transiently reduced in size due to interaction with ELMs is reported for the first time. Cold pulses due to ELMs result in peaking of Te at the O-point of core 2/1 islands. In sync the islands shrink. Next, the Te peak is rapidly relaxed via anomalous transport an the island heals. Ion-scale turbulence increases when Te is peaked which offers an explanation for the rapid relaxation of the Te peak. Therefore, these measurements suggest that turbulence accelerates NTM recovery after an ELM crash. Coupled predator-prey equations and modified Rutherford equation qualitatively replicate these observations. In this model, turbulence accelerates NTM recovery via restoring the flat Te profile and concomitant bootstrap current perturbation. The key physics of the relationship between the Te peak and NTM stability has potentially far-reaching consequences, such as NTM control via pellet injection in high-β tokamak plasmas.

The paper “Multi-field/-scale interactions of turbulence with neoclassical tearing mode magnetic islands in the DIII-D tokamak” was published in Phys. Plasmas (http://dx.doi.org/10.1063/1.4977533). This paper reports first measurements of simultaneous long (BES) and intermediate (DBS) wavelength turbulent density fluctuations and long wavelength turbulent Te fluctuations (CECE) interacting with m/n=2/1 NTMs. These correspond to the expected ITG and TEM scales, respectively. Two regimes have been observed: (1) small islands have steep Te radial profile and turbulence levels comparable to those of the background; (2) large islands are flat and have reduced turbulence level at the O-point. Radially outside the large island, Te is steeper and the turbulence level is increased during island growth. Qualitative comparisons to the GENE non-linear gyrokinetic code replicate the observed scaling of turbulence modification with island size. These results are significant as they allow the validation of gyrokinetic simulations modeling the interaction of these multi-scale phenomena.

The paper “Testing the role of molecular physics in dissipative divertor operations through helium plasmas at DIII-D” (http://dx.doi.org/10.1063/1.4982057) by Canik et al. has been published in Physics of Plasmas. This paper describes experiments and modeling studying divertor detachment in helium plasmas, and demonstrates that by reducing the role of chemical and molecular processes, 2D modeling codes can quantitatively reproduce the plasma parameters and radiative losses within the divertor volume. The work further highlights a missing contribution to pressure balance needed to relate the midplane and divertor parameters, indicating the value of improved ion temperature and flow measurements throughout the scrape-off layer.

The paper “Improved confinement in highly powered high performance scenarios on DIII-D” by Petrie et al. has been published in Nuclear Fusion (http://dx.doi.org/10.1088/1741-4326/aa7399). This paper reported on experiments that demonstrated significantly higher energy confinement in high power AT-class plasmas is possible under certain conditions and used the ELITE code to understand these results.

The paper “Effect of magnetic islands on profiles, flows, turbulence and transport in nonlinear gyrokinetic simulations” by A Bañón Navarro et al has been published in Plasma Phys. Control. Fusion (http://dx.doi.org/10.1088/1361-6587/aa557e). Static magnetic islands are implemented in nonlinear gyrokinetic simulations with the GENE code. The effect of the islands on profiles, flows, turbulence and transport and the scaling of these effects with respect to island size were investigated. A clear threshold island width was found, below which the islands have little or no effect while beyond this point the islands significantly perturb flows, turbulence and transport. These simulations aim to provide guidelines for interpreting experimental results by comparing qualitative trends in the simulations.

The article “OEDGE modeling for the planned tungsten ring experiment on DIII-D” by J.D. Elder et al. has been published in Nuclear Materials and Energy. A new model for tungsten erosion in OEDGE was developed which imports charge-state resolved carbon impurity fluxes and impact energies from a separate OEDGE run which models the carbon production, transport and deposition for the same plasma conditions as the tungsten simulations. These values are then used to calculate the gross tungsten physical sputtering due to carbon plasma impurities which is then added to any sputtering by deuterium ions; tungsten self-sputtering is also included. (http://doi.org/10.1016/j.nme.2017.03.039)

The joint EAST/DIII-D paper “Confinement improvement in the high poloidal beta regime on DIII-D and application to steady-state H-mode on EAST”, by Siye Ding et al. has been published in Phys. Plasmas (http://dx.doi.org/10.1063/1.4982058). This paper discusses the experimental and modeling investigations of high betap plasmas on DIII-D, showing that the large-radius ITB is maintained when the scenario is extended from q95~12 to 7, and from rapid to near-zero toroidal rotation. The physics mechanism of turbulence suppression is identified as the Shafranov shift, which sets a betap threshold at about 1.9 for the large-radius ITB formation. The paper also describes progress in adapting and extending the high betap scenario on EAST, a superconducting tokamak with an actively cooled ITER-like tungsten divertor. Recent experiments achieved a 61 s fully non-inductive H-mode with stationary ITB features, and demonstrated the capability to broaden the current profile using LHCD. The improved physics understanding and modeling capabilities are being used to develop advanced scenarios for the China Fusion Engineering Test Reactor.

A Letter titled “Prediction of Nonlinear Evolution of Energetic-Particle-Driven Instabilities” by Duarte et al., was published in Nuclear Fusion. The paper validates a proposed general criterion used to predict the emergence of chirping oscillations of unstable Alfvén eigenmodes in DIII-D. The model includes realistic eigenstructure, detailed phase-space dependences of the instability drive, stochastic scattering and Coulomb drag. It is found that fast ion micro-turbulence is often a key component that induces mode transition from the fixed-frequency to the chirping regime. This work increases confidence in the applicability of such criterion for predicting different nonlinear regimes in future experiments (https://doi.org/10.1088/1741-4326/aa6232).

A paper titled “SOLPS analysis of neutral baffling for the design of a new diverter in DIII-D” by Sang et al. has been published in Nuclear Fusion. SOLPS modeling has been applied to study effects of neutral baffling on divertor detachment. The relative importance of two key aspects of divertor baffle geometry: (i) divertor closure, and (ii) field-target angle, have been investigated. The paper reports analysis of the effect on plasma detachment of divertor geometry itself and confirms that achieving detachment will likely benefit from use of a slot-slant divertor shape. This work provides a basic direction for the divertor optimization in a tokamak. (https://doi.org/10.1088/1741-4326/aa6548).

“Advances in understanding of high-Z material erosion and re-deposition in low-Z wall environment in DIII-D” by R. Ding et al was published in Nuclear Fusion (http://doi.org/10.1088/1741-4326/aa6451). The paper presents the recent progress on understanding of high-Z material erosion and re-deposition in a mixed materials environment with a carbon wall in DIII-D, which show high potential for control of erosion. The sheath properties and background impurities have been found to play a critical role in determining high-Z material erosion. Experimentally, high-Z material erosion has been successfully suppressed with electrical biasing, as well as by local gas puffing. The controlling physics was identified using the 3D Monte Carlo code ERO.

A paper entitled “The inter-ELM tungsten erosion profile in DIII-D H-mode discharges and benchmarking with ERO+OEDGE modeling” by T. Abrams et al. has recently been published in Nuclear Fusion. This paper describes the first measurements of inter-ELM resolved tungsten sputtering in the DIII-D divertor via fast spectroscopic diagnosis of neutral tungsten (WI) emission lines. These measurements were benchmarked against the ERO+OEDGE codes and good agreement was found after incorporating charge-state resolved carbon impurity fluxes and a mixed-material surface model in the simulations. This work takes important steps towards developing a predictive understanding of the tungsten source rate near the strike points during high-performance H-mode discharges. Such an understanding is necessary to mitigate core contamination from high-Z divertor sources in ITER and beyond. (https://doi.org/10.1088/1741-4326/aa66b2)

A paper titled: “Improving fast-ion confinement in high-performance discharges by suppressing Alfven eigenmodes” by G.J. Kramer at al. was published in Nuclear fusion. In this paper, it is shown that the degradation of fast-ion confinement in steady-state DIII-D discharges is quantitatively consistent with predictions based on the effects of multiple unstable Alfven eigenmodes on beam-ion transport. Simulation and experiment show that increasing the radius where the magnetic safety factor has its minimum is effective in minimizing beam-ion transport. This is favorable for achieving high performance steady-state operation in DIII-D and future reactors. A comparison between the experiments and a critical gradient model, in which only equilibrium profiles were used to predict the most unstable modes, show that in a number of cases this model reproduces the measured neutron rate well. (https://doi.org/10.1088/1741-4326/aa6456).

The paper “Fast-ion transport by Alfven eigenmodes above a critical gradient threshold” W.W. Heidbrink et al. was published in Physics of Plasmas (http://dx.doi.org/10.1063/1.4977535). Multiple simultaneous Alfven eigenmodes (AEs) lead to overlapping wave-particle resonances and stochastic fast-ion transport in fusion grade plasmas. The behavior results in a sudden increase in fast-ion transport at a threshold that is well above the linear stability threshold for Alfven instability. A novel beam modulation technique, in conjunction with an array of fast-ion diagnostics, probes the transport by measuring the fast-ion flux in different phase-space volumes. Well above the threshold, simulations that utilize the measured mode amplitudes and structures predict a hollow fast-ion profile that resembles the profile measured by fast-ion D-alpha spectroscopy; the modelling also successfully reproduces the temporal response of neutral-particle signals to beam modulation.

The article “DiMES PMI research at DIII-D in support of ITER and beyond” by D.L. Rudakov et al has been published in Fusion Engineering and Design (http://dx.doi.org/10.1016/j.fusengdes.2017.03.007). This article provides a review of the last 4 years of DiMES and MiMES results and highlights experiments studying sputtering erosion, re-deposition and migration of high-Z elements, mostly tungsten and molybdenum, as well as some alternative materials. Two methods of high-Z PFC surface erosion control, with (i) external electrical biasing and (ii) local gas injection, are also discussed. Initial results on short and longer range W migration in L-mode part of the recent W tile mini-campaign are presented.

“Pedestal-to-wall 3D fluid transport simulations on DIII-D”, by J.D. Lore et al has been published in Nucl. Fusion (http://doi.org/10.1088/1741-4326/aa64ad). The 3D fluid plasma transport code EMC3-EIRENE was used to test several plasma response models under the application of n=3 3D magnetic fields against experimental data in the pedestal, SOL, and divertor. Plasma response is found to be required to reduce the stochasticity of in the pedestal region for even-parity perturbations, however if the screening is too strong the non-axisymmetric structure in the downstream data is suppressed. With odd-parity perturbations plasma response is required to increase the size of the lobe structure. No single model tested is able to simultaneously reproduce upstream and downstream experimental conditions for both parities.

The paper “Advances in the high bootstrap fraction regime on DIII-D towards the Q=5 mission of ITER steady state”, by J.P. Qian et al. has been published in the journal Nuclear Fusion (https://doi.org/10.1088/1741-4326/aa626a). Recent joint EAST/DIII-D experiments on the high poloidal beta (betaP) regime in DIII-D have extended operation with internal transport barriers (ITBs) and excellent energy confinement (H98y2 ~ 1.6) to higher plasma current (lower q95 ≤ 7.0), and more balanced neutral beam injection (NBI) (torque injection < 2 Nm), for lower plasma rotation than previous results. Transport analysis and experimental measurements at low toroidal rotation suggest that the E×B shear effect is not key to the ITB formation in these high betaP discharges. Experiments and TGLF modeling show that the Shafranov shift has a key stabilizing effect on turbulence. Extrapolation of the DIII-D results to ITER show that with some q-profile optimization, the high bootstrap fraction regime could achieve steady-state fusion gain Q=5 at betaN ~ 2.9 and q95 ~ 7.

Stability of DIII-D high-performance, negative central shear discharges, J.M. Hanson et al has been published in Nuclear Fusion (http://doi.org/10.1088/1741-4326/aa6266). Experiments have demonstrated high-performance, negative central shear (NCS) equilibria with enhanced stability when the minimum safety factor qmin exceeds 2, qualitatively confirming theoretical predictions of favorable stability in the NCS regime. The discharges exhibit good confinement with an L-mode enhancement factor H89 = 2.5, and are ultimately limited by the ideal-wall external kink stability boundary as predicted by ideal MHD theory, as long as tearing mode (TM) locking events, resistive wall modes (RWMs), and internal kink modes are properly avoided or controlled. Ideal MHD stability analysis predicts that the ideal-wall limit can be further increased to beta normal > 4 by broadening the current profile. This path toward improved stability has the potential advantage of being compatible with the bootstrap-dominated equilibria envisioned for advanced tokamak (AT) fusion reactors.

The paper, “Effect of rotation zero-crossing on single-fluid plasma response to three-dimensional magnetic perturbations”, by B C Lyons et al, was published in Plasma Phys. Control. Fusion (http://doi.org/10.1088/1361-6587/aa5860). A systematic scan of the zero-crossing of the rotation profile in M3D-C1 plasma response simulations confirmed that the resonant magnetic field generally increases as the rotation decreases at a rational surface. In addition, non-resonant current driven at zero-crossings was found to produce a quasilinear electromagnetic torque that could flatten the rotation and drive the zero-crossing toward rational surfaces. By one or both of these mechanisms, this torque may play an important role in bifurcations into ELM suppression. Finally, it was shown that the changes to the resonant field should be observable by magnetic sensors on the high-field side of tokamaks.

A paper entitled “A simplified analytic form for generation of axisymmetric plasma boundaries” by T. Luce was published in Plasma Physics and Controlled Fusion (https://doi.org/10.1088/1361-6587/aa5393). It describes a general analytic formula for characterizing plasma boundaries with and without x points in terms of standard shape quantities and more detailed shape metrics such as squareness. The formula can also be used to generate target boundaries either with or without x points for equilibrium calculations or targets for plasma control of real plasmas. Previously, no general method to generate boundaries with x points was available.

A paper “Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics” by Boedo and Rudakov has been published in Review of Scientific Instruments (Vol.88, Issue 3), in which a new method to calculate the ion saturation current Isat for Langmuir probes at high frequency (>100 kHz) using the harmonics technique is presented. They also demonstrated that since the probe tips using the harmonic method are oscillating near the floating potential drawing little power, this method reduces tip heating and arcing and allows plasma density measurements at a plasma power flux that would cause continuously biased tips to arc. http://dx.doi.org/10.1063/1.4978453

A paper titled “Predicting rotation for ITER via studies of intrinsic torque and momentum transport in DIII-D” has been published in Physics of Plasmas by C. Chrystal et al. This paper uses dimensionless parameter scans to predict a pedestal rotation for ITER and compare momentum transport measurements to TGLF simulations. Discharges that varied Te/Ti, q, and collisionality were used to compare measured and TGLF predictions of power balance and increments momentum transport. The measured incremental momentum transport is consistent with the ExB shear suppression of turbulent transport, and TGLF predictions were found to be more accurate at lower q and collisionality. Using TGYRO and TGLF, a prediction of the ITER rotation profile was made based on NBI torque in the core and a boundary condition in the pedestal from intrinsic torque scaling. Rotation in the core is about 20 krad/s (120 km/s), and the ExB shear is predicted to be large enough to significantly affect transport and the ITER fusion performance. (http://dx.doi.org/10.1063/1.4979194)

“Magnetic shear effects on plasma transport and turbulence at high electron to ion temperature ratio in DIII-D and JT-60U plasmas” by M. Yoshida et al has been published in Nuclear Fusion (https://doi.org/10.1088/1741-4326/aa611e). Negative magnetic shear (NCS) has been demonstrated in DIII-D and JT-60U to mitigate the confinement degradation typically observed with increasing Te/Ti in positive magnetic shear (PS) plasmas. The mechanism of the different transport responses between the NCS and PS plasmas has been assessed in terms of fluctuation measurements in DIII-D; NCS gave a smaller rise in the low-wavenumber broadband turbulent fluctuations with the increase in Te/Ti compared with the PS case, showing consistency with gyrokinetic simulations. 30

A paper Strong correlation between D2 density and electron temperature at the target of divertors found in SOLPS analysis by Stangeby and Sang has been published in Nuclear Fusion, https://doi.org/10.1088/1741-4326/aa5e27. An unexpectedly strong and simple correlation has been discovered in SOLPS code divertor analysis between the electron temperature, Tet, and the D2 density, nD2t, at the divertor target, for Tet < 10 eV and extending over 2 orders of magnitude for each correlate: Tet = 6.14e13/nD2t^0.68 with R^2 = 0.98. This may imply that achievement of low Tet reduces, essentially, to identifying the divertor baffle geometry that achieves the highest gas density near the target.

A paper titled “Dependence of intrinsic torque and momentum confinement on normalized gyroradius and collisionality in the DIII-D tokamak” has been published in Physics of Plasmas by C. Chrystal et al. This paper describes results from dimensionless parameter scans that investigate intrinsic rotation by measuring intrinsic torque and momentum confinement time. When normalizing intrinsic torque with the ion temperature, it is found to increase significantly with lower normalized gyroradius and decrease slightly with decreasing collisionality. Momentum confinement time, normalized by the Bohm time, shows a dependence that is not clearly Bohm of gyro-Bohm and a small increase with decreasing collisionality. These results will be combined with similar measurements from JET and ASDEX-U in the future. Based solely on the DIII-D results, the projected intrinsic torque for ITER is 33 Nm and nearly equal to ITER's expected NBI torque. (http://dx.doi.org/10.1063/1.4978563)

The paper, “A path to stable low-torque plasma operation in ITER with test blanket modules” by M.J. Lanctot et al was recently published in Nucl. Fusion 57 036004, (https://dx.doi.org/10.1088/1741-4326/57/3/036004). New experiments in the 15MA ITER Q=10 scenario on DIII-D at low rotation demonstrate that n = 1 magnetic fields from a single row of ex-vessel control coils enable operation at ITER performance metrics in the presence of applied non-axisymmetric magnetic fields from a test blanket module (TBM) mock-up coil. With n = 1 compensation, operation below the ITER-equivalent injected torque is successful at three times the ITER equivalent toroidal magnetic field ripple for a pair of TBMs in one equatorial port, whereas the uncompensated TBM field leads to rotation collapse, loss of H-mode and plasma current disruption. These results show that the n = 1 plasma response plays a dominant role in determining plasma stability at low rotation. Although an extrapolation of the DIII-D results to the ITER configuration remains to be completed, these results suggest the proposed limits on the amount of ferromagnetic material permitted in each TBM are reasonable, and increase confidence that ITER can achieve its scientific mission while developing critical blanket technology for DEMO.

A paper entitled “Scenario development for high poloidal beta, low torque plasma with qmin above 2 and large-radius internal transport barrier in DIII-D”, by Siye Ding et al., has now been published online, and is available at http://dx.doi.org/10.1088/0029-5515/57/2/022016. This paper describes work by a joint research team from the DIII-D and EAST tokamaks on the development of of high betap scenario towards the inductive operation at higher fusion performance in the joint DIII-D/EAST joint experiment. High fusion performance (G=0.16) plasmas in DIII-D with qmin>2 and reduced NBI torque (~3-5 N-m) are achieved, which can be potentially extrapolated to EAST. Experiment profiles, turbulence measurement and linear gyrokinetic analysis are shown in this paper for the transient formation of strong large-radius ITB (at rho~0.7), which leads to very high confinement (H89=3.5 or H98=2.1 with betaN~3.0). The large-radius ITB is also found to have a shielding (protecting) effect on the core plasma while isolating the perturbation due to ELM crash.

FY17-Q2: (16 papers described here)

The paper “Interaction of external n = 1 magnetic fields with the sawtooth instability in low-q RFX-mod and DIII-D tokamaks” by C. Piron et al was published in Nuclear Fusion (https://doi.org/10.1088/0029-5515/56/10/106012). External n=1 magnetic fields cause a reduction of both sawtooth amplitude and period in low-q RFX-mod and DIII-D plasmas. In RFX-mod, sawteeth are replaced by a stationary m = 1, n = 1 helical equilibrium without an increase in disruptivity. In DIII-D, sawtooth stabilization is correlated with a clear increase of the n = 1 plasma response, i.e. of coupling to the marginally stable n = 1 external kink. A reduction of the plasma toroidal rotation is also observed in both devices.

The article “MHD modeling of a DIII-D low-torque QH-mode discharge and comparison to observations” by Jacob King et al. has been published in the journal Physics of Plasmas (https://doi.org/10.1063/1.4977467). Extended-magnetohydrodynamic simulations with the NIMROD plasma-modeling code produce a saturated long-wavelength state where a bath of multiple unstable modes interact to provide energy and particle loss. This state is consistent with many aspects of DIII-D experiments. In particular, simulations saturate to steady-state only when the flow inferred from the experiment is included and the simulated particle transport is enhanced relative to the thermal transport. Both effects are observed in experiment.

The article “Small angle slot divertor concept for long pulse advanced tokamaks”” by H. Guo et al has been published in Nuclear Fusion (https://doi.org/10.1088/1741-4326/aa5b46). This letter describes a new divertor configuration featuring the integration of a small incident angle near the plasma strike point on the divertor target plate with a progressively opening slot (i.e. a closed slot structure flaring out from the strike point), called the small angle slot (SAS) divertor. SOLPS-EIRENE edge code analysis shows that SAS can achieve cold, dissipative/detached divertor conditions at relatively low values of plasma density at the outside midplane separatrix. Such a divertor may potentially provide a power and particle handling solution for long pulse advanced tokamaks.

A paper titled “Effect of scrape-off-layer current on reconstructed tokamak equilibrium” has been published in Physics of Plasmas by Jacob King et al. This paper investigates the effect of extending fields from reconstructed equilibria to include scrape-off-layer current through extrapolated parametrized and experimental fits. To quantify the degree that inclusion of scrape-off-layer current modifies the equilibrium, the χ-squared goodness-of-fit parameter is calculated for cases with and without scrape-off-layer current. The change in χ-squared is found to be minor when scrape-off-layer current is included; however, flux surfaces are shifted by up to 3 cm. The impact on the linear-stability of edge modes with these scrape-off-layer modifications is also found to be small and the importance of these methods to nonlinear computation is discussed. (http://doi.org/10.1063/1.4972822).

The paper “Calculation of the radial electric field from a modified Ohm's law” describes a reduced model for calculating the radial electric field in the DIII-D edge pedestal plasma from a modified Ohm's Law which takes into account neoclassical physics corrected by the kinetic effects of ion orbit loss through return currents and intrinsic rotation. The Ohm's law is shown to agree with representative DIII-D H-mode, RMP, and L-mode discharges, and works towards a predictive model for Er by evaluating the expression through analytical rotation models, with a key result highlighting the importance of predictive rotation models required for a predictive radial electric field calculation. The paper was recently published in Physics of Plasmas by T.M. Wilks et al (http://dx.doi.org/10.1063/1.4973599).

A paper, “Investigation of energy transport in DIII-D High-β P EAST-demonstration discharges with the TGLF turbulent and NEO neoclassical transport models”, has been published in the journal Nuclear Fusion by Chengkang Pan. This paper describes energy transport analyses of the DIII-D high-poloidal-beta, EAST-demonstration discharges performed using the TGYRO transport package with the TGLF turbulent and NEO neoclassical transport models. Ion energy transport is shown to be dominated by neoclassical transport and ion temperature profiles predicted by TGYRO agree closely with the experimental measured profiles. Various mechanisms for suppression of the ion turbulent energy transport are investigated. Electron turbulent energy transport is shown to be under-predicted by TGLF and a significant shortfall in the electron energy transport over the whole core plasma is found with TGLF predictions for these high-poloidal-beta discharges. TGYRO can successfully predict the experimental ion and electron temperature profiles by artificially increasing the saturated turbulence level for ETG driven modes used in TGLF. (http://dx.doi.org/10.1088/1741-4326/aa4ff8)

The paper, “The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress” by L. Schmitz et al was recently published in Nuclear Fusion (http://dx.doi.org/10.1088/1741-4326/57/2/025003). This paper reviews recent experimental and modeling progress in L- to H-mode transition physics and trigger dynamics. The coupling or edge turbulence and plasma flows via the turbulent Reynolds has been shown to play a crucial role in the transition across a range of experiments, and quantitative understanding of the microscopic physical picture of shear flow amplification stress has significantly advanced. Transitions have been simulated via simplified 0-D/1-D heuristic models and reduced 2-D/3-D fluid models at moderate and high collisionality. Present challenges and the need for developing more complete physics-based models predicting the L-H transition power threshold for burning plasma experiments are discussed.

The paper “Effect of thick blanket modules on neoclassical tearing mode locking in ITER” by R.J. La Haye et al has recently been published in Nucl. Fusion (http://dx.doi.org/10.1088/0029-5515/57/1/014004). The rotation of m/n=2/1 tearing modes can be slowed and stopped (i.e. locked) by eddy currents induced in resistive walls. This is a particular issue in ITER with large inertia and low applied torque. Previous estimates of tolerable 2/1 island widths in ITER found that the ITER electron cyclotron current drive (ECCD) system could catch and subdue such islands before they persisted long enough and grew large enough to lock. These estimates were based on a forecast of initial island rotation using the n=1 resistive penetration time of the inner vacuum vessel wall and benchmarked to DIII-D high-rotation plasmas. However, rotating tearing modes in ITER will also induce eddy currents in the blanket as the effective first wall that can shield the inner vessel. Recent DIII-D ITER baseline scenario (IBS) plasmas with low rotation through small applied torque allow better modeling and scaling to ITER with the blanket as the first resistive wall. The closer fitting blanket wall has a much shorter time constant and should allow several times smaller islands to lock several times faster in ITER than previously considered; this challenges the ECCD stabilization.

The paper “Results from Core-Edge Experiments in High Power, High Performance Plasmas on DIII-D” by T.W. Petrie, et al. has been published in Nuclear Materials and Energy (http://doi.org/10.1016/j.nme.2016.10.029). This paper covered several important issues: (1) The peak heat flux dependence on power input and plasma current in the near double-null configuration was shown to be consistent with ITPA predictions based on single-null shaping; (2) a route to improved energy confinement is accessible with deuterium gas puffing at very high power input in the DIII-D double-null configuration; and (3) the peak heat flux in the semi-slot divertor was significantly reduced compared with an “open” divertor and augurs well for a fully-enclosed slot.

Correction of MSE measurements to account for the effect of intrinsic radial electric fields in the plasma has required a labor-intensive off-line approach involving fitting profiles and re-running the EFIT magnetic reconstructions from data files. A new approach, using the approximation that the radial electric field can be estimated from the toroidal rotation alone, was described in the paper “Method for correction of measured polarization angles from motional Stark effect spectroscopy for the effects of electric fields” by Luce et al, appearing in Plasma Physics and Controlled Fusion. The method can be automated and use the toroidal rotation from the automated CER analysis, which allows a fairly accurate correction to the MSE data between shots or even in the real-time rtEFIT used for plasma control. The method was implemented directly inside EFIT and will be available for the upcoming operational campaign. (http://dx.doi.org/10.1088/0741-3335/58/12/125010)

A paper titled “​Improvements to an ion orbit loss calculation in the tokamak edge” was recently published by T.M. Wilks and W.M. Stacey (http://aip.scitation.org/doi/full/10.1063/1.4968219). The paper reports improvements to a reduced model calculating both thermal and fast ion orbit losses predicting intrinsic rotation peaking close to the separatrix, which is consistent with experimental data from DIII-D.​ ​A new mechanism of “x-transport pumping” is presented, which predicts larger ion losses when coupling conventional ion orbit loss and x-loss mechanisms, though losses are still dominated by conventional ion orbit loss.​ Sensitivity to these ion orbit loss model improvements is illustrated by fluid predictions of neoclassical rotation velocities and radial electric field profiles, with and without the enhancements.

A paper titled “NIMROD modeling of quiescent H-mode: reconstruction considerations and saturation mechanism” has been published in Nuclear Fusion by Jacob King. The paper explains the occurrence and saturation of broadband edge localized MHD, which acts to regulate the pedestal, thereby avoiding ELM instabilities, while still enabling high performance in so called “Quiescent H mode” discharges. This effect is due to growth and interaction of several low order modes in the plasma edge leading to a turbulent like state, as described by NIMROD extended-MHD modeling. Evaluation of the transport from the turbulent-like MHD state leads to turbulent density and temperature fluctuations which relax the profiles. (http://dx.doi.org/10.1088/0029-5515/57/2/022002).

A paper “Tungsten dust remobilization under steady-state and transient plasma conditions” by S. Ratynskaia et al has been published. The paper describes a systematic cross-machine study to investigate the remobilization of tungsten micron-size dust from tungsten surfaces in fusion plasmas under steady-state and transient conditions, using a newly developed technique based on controlled pre-adhesion of dust samples by gas dynamics methods. http://dx.doi.org/10.1016/j.nme.2016.10.021

A paper describing the principal results of Nathan Bolte's Ph.D. thesis, “Measurement and simulation of passive fast-ion D-alpha emission from the DIII-D tokamak” was published in Nuclear Fusion 56 (2016) 112023 (http://dx.doi.org/10.1088/0029-5515/56/11/112023). The ions that produce the FIDA light come from three distinct sources: ions that pass through the diagnostic sightlines on their first full orbit, an axisymmetric confined population, and ions that are expelled into the edge region by instabilities. The paper shows that passive FIDA measurements can be used to measure the edge neutral density and to estimate the number of ions expelled by bursting instabilities. In a sample discharge, approximately 1% of the fast-ion population is ejected into the high neutral density region per sawtooth crash.

A paper, “Avoidance of tearing mode locking with electro-magnetic torque introduced by feedback-based mode rotation control in DIII-D and RFX-mod”, has been published in the journal Nuclear Fusion by Michio Okabayashi. This paper explores how the use of rotating 3D fields can couple to locked modes and maintain this robust coupling through advanced feedback techniques. The technique was found robust on two very different devices and operating regimes, with both DIII-D and RFX-mod experiments showing effective application as well as remarkable consistency with theoretical predictions of torque balance. Application of the technique to avoid disruptions in ITER is discussed. (http://dx.doi.org/10.1088/1741-4326/57/1/016035).

The paper “SOLPS modeling of the effect on plasma detachment of closing the lower divertor in DIII-D” by C.F. Sang et al has been published in Plasma Physics and Controlled Fusion. In the paper, SOLPS modeling indicates that the tightly closed divertor greatly improves trapping of recycling neutrals, thereby increasing radiative and charge exchange losses in the divertor and reducing the electron temperature and deposited power density at the target plate. Furthermore, the closed structure enables the divertor plasma to enter into highly dissipative and detached divertor conditions at a significantly lower upstream density. (http://dx.doi.org/10.1088/1361-6587/59/2/025009)

A paper titled “Statistical analysis of m/n=2/1/ locked and quasi-stationary modes with rotating precursors at DIII-D” was recently published in Nuclear Fusion, by Ryan Sweeney, a Columbia University graduate student working on DIII-D. The paper reports on the analysis of a database of discharges with said modes and characterizes the average behavior of locked mode disruptions. It also identifies key parameters such as li/q95 that exhibit predictive capability over whether a mode leads to disruption, and explores how island overlap leads to the termination (http://dx.doi.org/10.1088/0029-5515/57/1/016019).

Simulations and experiments related to neutral beam prompt loss power striking the low power helicon antenna installation in DIII-D are presented in a recently published paper “Consideration of neutral beam prompt loss in the design of a tokamak helicon antenna”. A predicted peak power density of 2 MW/m^2 is confirmed by helicon antenna temperature measurements during the first experiments following installation. Understanding prompt losses is growing in importance as more tokamaks operate counter-injection beams. The paper is published as D.C. Pace, et al., Fusion Engineering Design 112, 14 (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.07.018

A paper “Use of Ar pellet ablation rate to estimate initial runaway electron seed population in DIII-D rapid shutdown experiments,” by E. Hollmann et al recently appeared in Nuclear Fusion. This paper used Ar-I line brightness to estimate the Ar pellet ablation rate during disruption runaway electron experiments. The ablation rate was found to be dominated by fast electrons, not thermal electrons. This fast electron ablation was used to make first experimental estimates of disruption runaway electron seed currents, arriving at values of around 10 kA. These seed currents are about 100 times larger than predicted by the Dreicer formula and about 100 times smaller than predicted by the hot tail formula for these experiments. http://dx.doi.org/10.1088/0029-5515/57/1/016008

A Letter, “Control of power, torque, and instability drive using in-shot variable neutral beam energy in tokamaks”, recently published in Nuclear Fusion presents initial results from DIII-D experiments applying in-shot variation of injected neutral beam energy. Following two years of engineering development, this first-ever demonstration used pre-programmed energy waveforms to produce continuously varying injected torque at fixed beam power. The drive for energetic ion instabilities was also observed to change as expected with the evolving beam energies. This neutral beam capability is now available for all DIII-D experiments. The paper appears as D.C. Pace, et al., Nucl. Fusion 57, 014001 (2017), http://dx.doi.org/10.1088/0029-5515/57/1/014001.

APS DPP Conference: Many members of the DIII-D team and the Theory group participated in the 58th Annual Meeting of the APS Division of Plasma Physics, which was held in San Jose, CA from October 31-November 4, 2016. 11 invited talks, 23 contributed orals, and 104 poster presentations were made, in addition to invited theory papers that made use of DIII-D data.

The paper “Experimental evidence of edge intrinsic momentum source driven by kinetic ion loss and edge radial electric fields in tokamaks,” by J. Boedo et al has been published in Physics of Plasmas 23, 092506 (2016), http://dx.doi.org/10.1063/1.4962683. The paper shows evidence that thermal ion loss makes an important contribution to the source for intrinsic torque in present tokamaks and that the edge electric field and potential fluctuations are important in determining the loss cone boundary. The paper also shows that externally injected torque by neutral beams can overwhelm the edge torque and determine the near SOL flows.

Development of a First-Principles Self-Consistent Core-Pedestal Model and its Application to ITER Meneghini USA

Kinetics of Relativistic Runaway Electrons Breizman USA

EAST oral: Development of high poloidal beta, steady-state scenario with ITER-like W divertor on EAST Garofalo USA

A recent Nuclear Fusion paper “Stationary QH-mode plasmas with high and wide pedestal at low rotation on DIII-D” by Xi Chen, et al (http://dx.doi.org/10.1088/0029-5515/57/2/022007) examines the physics of the new low torque wide-pedestal QH regime. As the neutral beam torque ramps down and the edge E × B rotation shear reduces, a transition occurs where the coherent EHO that regulates the standard QH edge ceases and broadband edge MHD modes appear along with a rapid increase in the pedestal height (by ⩽60%) and width (by ⩽50%). It is posited that the enhanced edge turbulent transport, enabled by the lower edge E × B flow shear, reduces the pedestal gradient and, combined with the high edge stability limit provided by the balanced double-null plasma shape, permits the development of a broader and thus higher pedestal. The improved transport in the outer core region (0.7<rho<0.9) is consistent with the enhanced global confinement

A Physical Review Letter article “Evidence of Toroidally Localized Turbulence with Applied 3D Fields in the DIII-D Tokamak”, by R.S. Wilcox, M.W. Shafer et al. (http://dx.doi.org/10.1103/PhysRevLett.117.135001), by ORNL postdoc Robert Wilcox, demonstrates that 3D fields lead to widespread particle redistribution within flux surfaces and significant asymmetries in plasma turbulence. Increased broadband edge density fluctuations observed with beam emission spectroscopy are correlated with an increase in the density gradient measured using profile reflectometry. Simulation with M3D-C1 reveals these density variations to be a two fluid effect arising within flux surfaces in response to the applied 3D field. The resulting density gradient variation is hypothesized to drive the observed turbulence rises. These mechanisms lead to significant changes in edge particle transport and may explain the so called density pump-out effects observed with 3D fields are applied for purposes such as ELM control.

The paper “Snowflake Divertor Experiments in the DIII-D, NSTX, and NSTX-U Tokamaks Aimed at the Development of the Divertor Power Exhaust Solution” by V. A. Soukhanovskii et. al. was published in IEEE Transactions on Plasma Science (Volume: 44, Issue: 12, Dec. 2016, p. 3445; DOI: 10.1109/TPS.2016.2625325). The manuscript was based on material presented in an invited talk at the Symposium of Fusion Engineering 2015. The paper summarized experimental results obtained in NSTX and DIII-D: divertor heat flux reduction, radiation distribution and full power detachment in the radiative snowflake divertor, effects on pedestal profiles and ELM energy, and significant divertor surface peak temperature reduction during ELMs. Magnetic equilibria, divertor transport and radiative divertor modeling results for NSTX-U support the snowflake divertor configuration as a leading divertor heat flux mitigation scenario for 2MA, 12 MW NBI-heated H-mode plasmas.

The paper “Developing and validating advanced divertor solutions on DIII-D for next-step fusion devices” by H. Guo et al has been published in Nuclear Fusion (http://dx.doi.org/10.1088/0029-5515/56/12/126010). This article highlights recent progress in boundary/PMI research on DIII-D, and discusses future plans toward developing a viable boundary solution for next-step steady-state fusion devices.

A manuscript on “Asymmetries in the motional Stark effect emission on the DIII-D tokamak” by B. S. Victor et. al. was published in Review of Scientific Instruments (http://dx.doi.org/10.1063/1.4961560). This manuscript reports on asymmetries that arise in the Stark-split D_alpha emission due to the ellipticity of the emitted light across the Stark components coupled with imperfect polarization preservation from an in-vessel mirror.

“High resolution main-ion charge exchange spectroscopy in the DIII-D H-mode pedestal” by B.A. Grierson et. al. recently appeared in Review of Scientific Instruments (http://dx.doi.org/10.1063/1.4960604). A new high spatial resolution main-ion (deuterium) CER system has been installed on DIII-D and is collecting detailed measurements of the main-ion temperature, velocity and density spanning the plasma boundary from the top of the H-mode pedestal into the scrape-off layer. The article presents the diagnostic and measurements in a type-I ELMing H-mode and demonstrates that the main-ion velocity can be substantially different than the more commonly measured impurity carbon. This new capability will be used in an upcoming research thrust on understanding momentum transport near the H-mode edge and the origin of intrinsic rotation.

A manuscript on “Validation of conducting wall models using magnetic measurements” was published in Nuclear Fusion (http://dx.doi.org/10.1088/0029-5515/56/10/106022 ). The manuscript addresses key issue for understanding the measurement and control of tokamak stability to long-wavelength MHD modes: the impact of conducting wall eddy currents on measurements of the perturbed magnetic field. The manuscript details a validation study for the wall models in VALEN and MARS-F codes, using frequency-dependent measurements of the vacuum couplings between the DIII-D I-coils and magnetic sensors.

“Thermal quench mitigation and current quench control by injection of mixed species shattered pellets in DIII-D” by D. Shiraki et al, was published in Physics of Plasmas (http://dx.doi.org/10.1063/1.4954389). Injection of shattered pellets composed of variable quantities of deuterium and neon allows the tuning of thermal quench (TQ) and current quench (CQ) properties during mitigated disruptions. By varying the relative quantities of the two species in the pellet mixture, TQ radiation fractions, CQ rates, and resulting halo current fractions can be smoothly varied. These disruption loads are found to saturate as a function of the neon quantity, within scaled injection quantities anticipated for ITER, providing a possible technique for tuning disruption loads to remain within allowable limits.

A paper “Improved edge charge exchange recombination spectroscopy in DIII-D,” Rev. Sci. Instrum. 87 11E512 (2016), by C. Chrystal, K.H. Burrell, B.A. Grierson, S.R. Haskey, R.J. Groebner, D.H. Kaplan, and A. Briesemeister, has been published online at http://dx.doi.org/10.1063/1.4958915. This paper describes recent upgrades to the impurity CER diagnostic on DIII-D. The number of high resolution tangential and vertical views near the outboard midplane edge were increased from 16 to 38, allowing detailed pedestal measurements to be made for many different plasma shapes. The increased light throughput of the new fibers makes the observation of non-Gaussian spectrum apparent, and the observation of these spectra is correlated with reduced collisionality, which is consistent with the ion distribution function being non-Maxwellian.

In a recent Nuclear Fusion article by M.A. Van Zeeland, “Electron cyclotron heating (ECH) can drastically alter reversed shear Alfvén Eigenmode (RSAE) activity in DIII-D through finite pressure effects” (http://dx.doi.org/10.1088/0029-5515/56/11/112007), localized electron cyclotron heating (ECH) is found to have a dramatic impact on neutral-beam-driven Alfvén eigenmode (AE) activity in DIII-D reversed magnetic shear plasmas, with the actual effect depending sensitively on ECH deposition location, current ramp rate, ECH injection timing and beam power. The most commonly observed effect is a shift in the dominant modes from a mix of reversed shear Alfvén eigenmodes (RSAEs) and toroidicity induced Alfvén eigenmodes (TAEs) to a spectrum of weaker TAEs when ECH is deposited near the shear reversal point. Recent work has found that modification of the RSAE through finite pressure effects via the ECH impact on the local electron temperature and temperature gradient at qmin is the dominant factor changing the AE spectrum in these experiments. The impact on RSAEs is captured in a large collection of discharges by a simple model which is based on constraining the RSAE frequency sweep to be between some minimum frequency (fRSAE−min) and the TAE frequency (fTAE), where fRSAE−min is shifted above the local GAM frequency by an amount that depends on gradients of the thermal plasma. NOVA and TAEFL simulations confirm this interpretation and show that for cases with no observed RSAE, or much reduced sweep range, the typical RSAE is no longer an eigenmode of the system.

“Progress toward steady-state tokamak operation exploiting the high bootstrap current fraction regime” by Qilong Ren et al, recently appeared in Phys. Plasmas (http://dx.doi.org/10.1063/1.4948724). These joint DIII-D and EAST experiments have developed a fully non-inductive scenario on DIII-D that could be extended on EAST to a demonstration of long pulse steady-state tokamak operation. Improved understanding of the stability of the scenario has led to the achievement of very high values of βp and βN, with large radius internal transport barriers. Good confinement has been achieved also with reduced toroidal rotation. These experiments challenged our understanding of electron energy transport and provided a first test of the new multi-scale zonal-flow-mediated saturation model (SAT1) implemented in the quasilinear TGLF turbulent transport model (http://dx.doi.org/10.1063/1.4954905). The test yielded improved electron transport predictions for these high βp plasmas.

“Rotational shear effects on Edge Harmonic Oscillations in DIII-D quiescent H-mode discharges” by Xi Chen et al, recently appeared in Nuclear Fusion (http://dx.doi.org/10.1088/0029-5515/56/7/076011). A detailed theoretical, experimental (Magnetics, ECE, BES, ECE-I, MIR) and M3D-C1 modeling comparison has been made of low-n (n≤5) EHO in DIII-D QH-mode plasmas. Numerical investigations indicate that the low-n EHO-like solutions from M3D-C1 are destabilized by the rotational shear while high-n modes are stabilized. The modeling results are consistent with observations of the EHO, support the proposed theory of the EHO as a rotational shear driven kink/peeling mode, and improve our understanding and confidence in creating and sustaining QH-mode in present and future devices.

In a paper titled “Interpretation of fast-ion signals during beam modulation experiments” by Heidbrink, Collins, Stagner, Zhu, Petty and Van Zeeland, Nucl. Fusion 56 (2016) 112011, http://dx.doi.org/10.1088/0029-5515/56/11/112011, fast-ion signals produced by a modulated neutral beam provide a powerful new technique to probe phase-space details of fast-ion transport by instabilities. The measured quantity is the divergence of perturbed fast-ion flux from the phase-space volume measured by the diagnostic. Since velocity-space transport often contributes to this divergence, the phase-space sensitivity of the diagnostic (or “weight function”) plays a crucial role in the interpretation of the signal. The source and sink make major contributions to the signal but their effects are accurately determined. Heidbrink, Collins, Stagner, Zhu, Petty and Van Zeeland, Nucl. Fusion 56 (2016) 112011, http://dx.doi.org/10.1088/0029-5515/56/11/112011

A paper “High-beta, steady-state hybrid scenario on DIII-D” by C.C. Petty et al recently appeared in Nuclear Fusion (http://dx.doi.org/10.1088/0029-5515/56/1/016016). This paper examines the stability, transport and current profile properties of a ~100% non-inductive hybrid scenario with central current drive. A 0D physics model shows that similar steady-state hybrid operation with Q_fus~5 is feasible in FDF and ITER.

“Equilibrium drives of the low and high field side n = 2 plasma response and impact on global confinement”, by C. Paz-Soldan et al, recently appeared in Nuclear Fusion (http://dx.doi.org/10.1088/0029-5515/56/5/056001). In this work the effect of plasma beta, safety factor, and collisionality, on the low and high-field-side magnetic response is studied experimentally and computationally. Measurement showed that the LFS response was primarily sensitive to plasma beta, while the HFS response was primarily sensitive to collisionality. Measurement and modeling agreed that the low-field-side response was mostly due to the well-known pressure driven kink mode, while surprisingly most HFS experimental trends were not captured.

A paper on “High frequency pacing of edge localized modes by injection of lithium granules in DIII-D H-mode discharges” was recently published by Alessandro Bortolon et al. (http://dx.doi.org/10.1088/0029-5515/56/5/056008). Robust ELM pacing was demonstrated in ITER-like plasmas at ELM frequencies 3–5 times larger than the 'natural' ELM frequency observed in reference discharges. Within the range of ELM frequencies obtained, the peak ELM heat flux at the outer strike point was reduced with increasing pacing frequency. Overall, high frequency ELM pacing using the lithium granule injection appears compatible with both H-mode energy confinement and attractive H-mode pedestal characteristics, but further assessment is needed to determine whether the projected heat flux reduction required for ITER can be met.